Planet Building 7: Tropical Cyclones

What time of year? Marna frowned a little. She had noticed the days were getting shorter when—over a month ago?–and since she wasn’t living by clocks and calendars, the actual solstice had probably been considerably before that. Late summer, then. She looked again at the sky to the south, and felt a sudden chill that had nothing to do with the wind. Tropical storm season. She’d forgotten about that, too, living on the satellite. She reached southward with her mind, and gulped at the intensity of the swirling winds she felt. She was only on the outermost fringe of the storm, and already her little boat was close to its limits. If the storm moved north …. From Homecoming, by Sue Ann Bowling

Is it going too far, to have hurricanes on another planet?

No, not if the conditions for tropical cyclones are present.

Three conditions are critical. First, the planet must have liquid water in fairly large quantities—oceans, in fact. Second, the planet must be rotating. Third, the temperature of the water must be high enough. The actual paths and frequencies of tropical cyclones are strongly influenced by details of the atmospheric circulation, but these three are critical for a planet to have tropical cyclones–also known as hurricanes and typhoons–at all.

The need for water is fairly obvious—you can’t have clouds without water.

The need for the planet to be rotating may not be as obvious, but on a rotating planet things moving on the surface of the planet do not (except at the equator) move in straight lines. If the north pole of the planet is defined so that sunrise is on the right hand when facing north, anything moving in the northern hemisphere is deflected to the right. In the southern hemisphere, deflection is to the left. Because of this deflection, called the Coriolis effect, air moving toward low pressure spirals in rather than flowing directly toward low pressure.

Among other things, the rotation of our planet causes the spiral structure of a hurricane as seen from space. Remember this does not happen right at the equator, and in fact tropical cyclones cannot form at the equator. On Earth, the earliest development of a tropical cyclone takes place at least 300 miles from the equator. On a planet that rotated faster than earth they could form closer to the equator; on a planet rotating more slowly, they would need to form closer to the poles.

This is where temperature comes in. It turns out that when the water temperature is 80 degrees F or warmer, and winds are accelerating evaporation by whipping up the water’s surface, the rate of evaporation, and thus the transfer of latent heat to the air, provides enough energy to speed up the wind and increase the evaporation. In order to keep this process going, the depth of the warm water must be over 150 feet.

This is pretty warm water. In fact, warm, deep water far enough from the equator to allow hurricanes to form is found only after the summer heating period—summer and fall.

There are other conditions. The instability of the air is important, as is mid-level humidity, the lack of vertical wind shear so the top of the developing storm is not separated from the base, and an initial area of disturbance. But the temperature of the water is critical, and is the reason hurricanes lose force so rapidly when they move over land.

Does this mean that global warming will increase hurricanes and related storms such as typhoons? Possibly. It will almost certainly increase the area over which tropical storms can form—the area with water warm enough and deep enough more than 300 miles from the equator. But other changes, more difficult to predict, may also affect hurricane formation and steering.

At any rate, rotating planets with oceans and with temperatures similar to or warmer than the Earth would be quite likely to have hurricane-like storms.